This invention relates to an optimized power plant for vehicle propulsion or stationary power production. In particular, the power plant relates to an energy optimization and recovery system for a high pressure thermal engine operating under attenuated load conditions for maximizing fuel efficiency.
Modern reciprocal engines operate at high pressure to maximize the efficiency of converting thermal energy to mechanical work. Unfortunately, conventional high pressure engines are characterized by a pronounced "hook shape" in the graph of specific fuel consumption under changing load conditions. At loads higher or lower than the "optimum point," fuel efficiency rapidly degenerates. The ordinary driving conditions for a family vehicle provide a wide range of operating loads, and the vehicle engine is rarely operating at optimum efficiency.
To gain the advantage of high engine efficiency and yet accommodate varying load conditions of ordinary driving, modern power plant designers for vehicles are turning to hybrid propulsion systems. By coordinating engine operation with power from a stored energy reserve, load demands are attenuated allowing engine operation within a narrower optimum range.
Certain solutions for boosting the efficiency of a thermal engine in a hybrid propulsion system have been proposed, and reference is herein made to our copending applications, Ser. No. 08/563,923, filed 29 Nov. 1995, entitled ADVANCED THERMOELECTRIC SYSTEMS FOR HYBRID ELECTRIC VEHICLES.
Typically, to achieve high operating pressures, engines are supercharged by a turbocharger which is a turbine driven compressor that delivers air to the engine at an optimal pressure for the engine load. However, typical turbine driven compressors also have a narrow range of efficient operation, and under higher engine loads, a waste gate is employed to prevent over-compression. A waste gate allows a portion of expanding exhaust gases from the engine to by-pass the turbocharger and be vented without generating useful work. Frequently, long-haul trucks and passenger vehicles spend a high percentage of travel time on freeways, where valuable thermal energy during this high load, high rotation operation would be lost, drastically reducing the overall fuel efficiency of a turbocharged engine.
A stored energy reserve may comprise a flywheel, a high-density capacitor or more conventionally a plurality of batteries forming a battery bank. Because of the cost and inherent weight of a battery bank, or a combination-type energy reserve, it is desirable that the capacity and hence the size and weight be minimized. Therefore, to achieve the advantages of a hybrid propulsion system, where electrical drive motors supplement or are supplied electricity by the power output of the thermal engine, the stored energy reserve is reduced in capacity by the supplemental electrical energy recoverable in the novel power plant of this invention. The stored energy reserve, for example, can be sized to accommodate storage of recovered energy from brake regeneration systems that transform deceleration of the vehicle into electrical power.
Since the direct recovery of thermal energy and its transformation into electrical energy in the invented system occurs under high demand and output conditions, the converted energy is immediately placed in service to supplement the power output of the thermal engine. There is little or no requirement that the storage capacity of the energy reserve be expanded to accommodate any carryover, except to replenish a depleted reserve.
Similarly, in stationary power generating plants, the recovery of the thermal energy ordinarily lost in a waste gate is immediately recovered as supplementary power, thereby reducing the size of the thermal engine required for a particular service.
In each case, the compressor delivering air to the thermal engine operates at optimum efficiency and is driven by the required expansion gas stream to maintain optimization. Excess combustion gases are separated at the engine for passage through an auxiliary turbine that drives an electrical generator for the capture of otherwise lost thermal energy, as electrical energy.